Patent Application
20080137343
The invention generally relates to visual displays and more specifically to a lightsource for a head mounted display that uses a large aperture, small size lightsource.
Head-Mounted Displays (HMD) are a class of image display devices that can be used to display images such as those from television, digital versatile discs, computer applications, game consoles, and other similar applications. An HMD can be monocular (a single image viewed by one eye), biocular (a single image viewed by both eyes), or binocular (a different image viewed by each eye). Further, the image projected to the eye(s) may be viewed by the user as complete or as superimposed on the user's view of the outside world. Effective HMD designs typically account for providing a lightsource that will adequately illuminate the image seen by the HMD user.
To be effective as a lightsource in an HMD, a proposed design should be sufficiently small in size, should produce relatively bright and homogenous light, and should provide tri-color (red, green, blue, also know as “RGB”) illumination with minimal coloration effects. (In most HMD applications, tri-color illumination refers to red, green, and blue lightsources being at different positions. As such, the angle and position of the resulting light distribution may vary according to color.) However, the smaller a lightsource design becomes, the more difficult it is for that lightsource to produce quality light.
Typical backlight designs are geared to large scale applications. Lightsources are used to illuminate relatively large surfaces, such as LCD-monitors and microdisplays. For example, these displays are illuminated by emitting light from a large area of the lightguide so that light is diffused over its entire surface. As a result, light is homogenous, but has low brightness. This is generally acceptable for applications that do not require intense light. However, HMD applications typically require intense light dispersed over a much smaller surface area of the lightsource. Accordingly, there is a need for a lightsource that is small enough to be used in an HMD that produces bright, homogeneous light with minimal coloration effects.
Some arrangements attempt to improve light brightness and homogeneity in small scale backlight applications by manipulating microprisms within the lightguide (a process generally referred to as “depth modulation”). These arrangements typically vary the height of individual microprisms and the spacing between microprisms, to specifically direct light within the guide and improve brightness and homogeneity. Unfortunately, this requires complex microprism structures, which increases complexity in both the design and assembly of the lightsource.
Another problem relating to backlight-type lightsources relates to loss of light during transmission, as light may be lost when the microprisms direct light outside of a desired range. In some cases, efficiency is lost as light reflected from the microprism structure is reflected outside of an aperture relied upon to facilitate transmission of the light from the lightguide to an illuminated body.
The present invention combines light reflected within a lightguide with direct light emitted from a lightsource and directs the combined light for use, thereby improving the overall quality of light emitted from the lightsource. Embodiments of the invention may accomplish this by providing a reflective backwall within the lightguide. This reflective backwall allows direct light, emitted from an illumination source, and the reflected light to combine within the lightguide, thereby increasing homogeneity and brightness. Also, reflective materials are strategically placed within the lightguide to improve efficiency of the transmission of light. As a result, the lightsource improves the quality of emitted light and provides homogenous light, both in position and angle-space, having high brightness. Moreover, the lightsource is of a dimension suitable to be employed in an HMD.
In an exemplary embodiment, an illumination unit and a lightguide are positioned with respect to one another where light propagates in a lightguide at a cone angle determined by the angle of Total Internal Reflection (TIR). The lightguide is of substantially rectilinear form, having side walls, a back wall, a bottom wall, and a top wall. The top wall has an aperture therein. On the bottom wall there is a microprism array. As light propagates through the lightguide, a portion is reflected within the lightguide by the side walls. The portion of light incident upon the rear wall, which comprises reflective material, is reflected within the lightguide. As a result, direct light from the illumination unit is combined with reflected light from the back wall by the microprism array on the bottom surface, thereby providing increased light brightness and homogeneity within the lightguide.
The microprism array that is symmetrical about a central portion of the lightguide. The prism angle is chosen so that light propagating at, or approximately equal to, one half of the angle of TIR is directed substantially perpendicular to the lightguide top surface aperture. Light incident upon the microprism array, which comprises both direct and reflected light, is reflected in a direction substantially perpendicular to the lightguide top surface aperture. As such, the combined light is effectively transmitted through the aperture. That portion of light reflected by the microprism array outside of the aperture is reflected by material surrounding the aperture. Thereby, the reflected light is given subsequent opportunity to later be transmitted through the aperture, which increases the overall efficiency of light transmission from the lightsource.
Referring to
As light propagates within lightguide 12 at the angle of TIR, it is reflected by the sidewalls towards end-wall 17. When the light reaches the end-wall, it is reflected within lightguide 12. The overall effect is an increase in brightness and homogeneity of light within lightguide 12. That is, brightness and homogeneity are increased as direct light, emitted from illumination unit 14, is combined with light reflected from end-wall 17. Increasing homogeneity within the lightguide 12 by virtue of reflective end-wall 17 allows microprism array 18 to be relatively simple in design. This is in contrast with typical backlight-type sources, which require depth modulation, or manipulation of height and spacing of individual microprisms, to improve the light brightness.
In the embodiment of
As discussed above, embodiments of lightsource 10 are further characterized by illumination unit 14. In the illustrated embodiment, illumination unit 14 comprises one or more Light Emitting Diodes (LED). LEDs are desirable in that they provide sufficient RGB illumination while being of a suitably small size. Those skilled in the art will recognize that illumination unit 14 may comprise other components. For example, incandescent emitters are thought to also be useful in some embodiments.
Referring now to
Referring now to
As seen in
Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.
